Synthesis of graphene/vitamin C template-controlled polyaniline nanotubes composite for high performance supercapacitor electrode

A graphene nanosheet/polyaniline nanotube (GPNT) composite is prepared for the first time by in-situ chemical oxidative polymerization of aniline using vitamin C as a structure directing agent. The vitamin C molecules lead to the synthesis of polyaniline (PANI) nanotubes through the development of rod-like assembly by H-bonding in an aqueous medium. The initially synthesized graphene oxide/polyaniline nanotubes composite is reduced to graphene using hydrazine monohydrate followed by re-oxidation and protonation of the PANI to produce the GPNT nanocomposite. This novel composite showed a high specific capacitance of 534.37 F/g and an excellent energy density of 74.27 Wh/kg at a constant current of 0.5 mA. Besides, the GPNT composite exhibited excellent cycle life with 91.4% specific capacitance retained after 500 charge-discharge cycles. The excellent performance is due to the synergistic combination of graphene which provides good electrical conductivity and mechanical stability, and PANI nanofiber which deals with good redox activity.     

Facile synthesis and strongly microstructure-dependent electrochemical properties of graphene/manganese dioxide composites for supercapacitors

Graphene has attracted much attention since it was firstly stripped from graphite by two physicists in 2004, and the supercapacitor based on graphene has obtained wide attention and much investment as well. For practical applications of graphene-based supercapacitors, however, there are still many challenges to solve, for instance, to simplify the technological process, to lower the fabrication cost, and to improve the electrochemical performance. In this work, graphene/MnO₂ composites are prepared by a microwave sintering method, and we report here a relatively simple method for the supercapacitor packaging, i.e., dipping Ni-foam into a graphene/MnO₂ composite solution directly for a period of time to coat the active material on a current collector. It is found that the microwave reaction time has a significant effect on the microstructure of graphene/MnO₂ composites, and consequently, the electrochemical properties of the supercapacitors based on graphene/MnO₂ composites are strongly microstructure dependent. An appropriately longer microwave reaction time, namely, 15 min, facilitates a very dense and homogeneous microstructure of the graphene/MnO₂ composites, and thus, excellent electrochemical performance is achieved in the supercapacitor device, including a high specific capacitance of 296 F/g and a high capacitance retention of 93% after 3,000 times of charging/discharging cycles.

PACS

81.05.ue; 78.67.Sc; 88.80.fh     

Self-standing flexible cathode of V2O5 nanobelts with high cycling stability for lithium-ion batteries

Self-standing V₂O₅ nanobelt electrode free of binders, conductive carbon or current collectors was successfully prepared via a simple one-step hydrothermal reaction. The length of V₂O₅ nanobelts was up to several hundreds micrometers and the thickness was around 40 nm. Ultralong nanobelts as building blocks and internal voids provide a robust mechanical flexibility and shortened ion/electron transport pathway. The self-standing electrode delivered an initial specific capacity of 127.4 mA h g⁻¹ at a current density of 60 mA g⁻¹ and exhibited excellent cycling stability with capacity retention up to 89.8% after 200 cycles. The outstanding cycling performance can be attributed to the excellent network stability, shortened Li-ion diffusion pathway and the high surface area between electrolyte/electrode interfaces.     

A novel method to prepare nanostructured manganese dioxide and its electrochemical properties as a supercapacitor electrode

Nanostructured MnO₂ was synthesized by co-precipitation in the presence of Pluronic P123 surfactant and characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscope (SEM) and transmission electron microscope (TEM). The sample without surfactant was spherical with particle size on the submicron scale, whereas P123-assisted samples were all loose clew shapes, consisting of MnO₂ nanowires, 8-20 nm in diameter and 200-400 nm in length. The electrochemical performances of the as-prepared MnO₂ as the electrode materials for supercapacitors were evaluated by cyclic voltammetry and galvanostatic charge-discharge measurements in a solution of 1 M Na₂SO₄. The sample without surfactant exhibited a relatively low specific capacitance of 77 F g⁻¹, whereas the nanostructured MnO₂ prepared with 0.02% (wt%) P123 exhibited excellent pseudocapacitive behavior, with a maximum specific capacitance of 176 F g⁻¹.     

Factors influencing MnO2/multi-walled carbon nanotubes composite's electrochemical performance as supercapacitor electrode

Poor crystallined α-MnO₂ grown on multi-walled carbon nanotubes (MWCNTs) by reducing KMnO₄ in ethanol are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and Brunauer-Emmett-Telle (BET) surface area measurement, which indicate that MWCNTs are wrapped up by poor crystalline MnO₂ and BET areas of the composites maintain the same level of 200 m² g⁻¹ as the content of MWCNTs in the range of 0-30%. The electrochemical performances of the MnO₂/MWCNTs composites as electrode materials for supercapacitor are evaluated by cyclic voltammetry (CV) and galvanostatic charge-discharge measurement in 1 M Na₂SO₄ solution. At a scan rate of 5 mV s⁻¹, rectangular shapes could only be observed for the composites with higher MWCNTs contents. The effect of additional conductive agent KS6 on the electrochemical behavior of the composites is also studied. With a fixed carbon content of 25% (MWCNTs included), MnO₂ with 20% MWCNTs and 5% KS6 has the highest specific capacitance, excellent cyclability and best rate capability, which gives the specific capacitance of 179 F g⁻¹ at a scan rate of 5 mV s⁻¹, and remains 114.6 F g⁻¹ at 100 mV s⁻¹.     

Nanocomposites of manganese oxides and carbon nanotubes for aqueous supercapacitor stacks

Symmetrical supercapacitors and their serially connected two-cell stacks via a bipolar electrode were constructed with nanocomposites of manganese oxides and carbon nanotubes (MnO_x/CNTs) as the electrode materials. Nanocomposites with different contents of MnO_x were synthesised through the redox reaction between KMnO₄ and CNTs in aqueous solutions. The nanocomposites were characterised by scanning and transmission electron microscopy, BET nitrogen adsorption and X-ray diffraction before being examined in a three-electrode cell with a novel trenched graphite disc electrode by electrochemical means, including cyclic voltammetry, galvanostatic charging-discharging, and electrochemical impedance spectroscopy. The nanocomposites demonstrated capacitive behaviour in the potential range of 0-0.85 V (vs Ag/AgCl) in aqueous KCl electrolytes with less than 9% capacitance decrease after 9000 charging-discharging cycles. Symmetrical supercapacitors of identical positive and negative MnO_x/CNTs electrodes showed capacitive performance in good agreement with the individual electrodes (e.g. 0.90 V, 0.53 F, 1.3 cm²). The bipolarly connected two-cell stacks of the symmetrical cells exhibited characteristics in accordance with expectation, including a doubled stack voltage and reduced internal resistance per cell.     

The additive-free electrode based on the layered MnO2 nanoflowers/reduced,graphene oxide film for high performance supercapacitor

The MnO₂ nanoflowers/reduced graphene oxide composite is coated on a nickel foam substrate (denoted as MnO₂ NF/RGO @ Ni foam) via the layer by layer (LBL) self-assembly technology without any polymer additive, following the soft chemical reduction. The layered MnO₂ NF/RGO composite is uniformly anchored on the Ni foam skeleton to form the 3D porous framework, and the interlayers have access to lots of ions channels to improve the electron transfer and diffusion. This special construction of 3D porous structure is beneficial to the enhancement of electrochemical property. The specific capacitance is up to 246 F g⁻¹ under the current density of 0.5 A g⁻¹. After 1000 cycles, it can retain about 93%, exhibiting excellent cycle stability. The electrochemical impedance spectroscopy measurements confirm that MnO₂ NF/RGO @ Ni foam electrode has lower R_ESR and R_CT values when compared to MnO₂ @ Ni foam and RGO @ Ni foam. This study opens a new door to the preparation of composite electrodes for high performance supercapacitor.     

A flexible graphene/multiwalled carbon nanotube film as a high performance electrode material for supercapacitors

A flexible graphene/multiwalled carbon nanotube (GN/MWCNT) film has been fabricated by flow-directed assembly from a complex dispersion of graphite oxide (GO) and pristine MWCNTs followed by the use of gas-based hydrazine to reduce the GO into GN sheets. The GN/MWCNT (16 wt.% MWCNTs) film characterized by Fourier transformation infrared spectra, X-ray diffraction and scanning electron microscope has a layered structure with MWCNTs uniformly sandwiched between the GN sheets. The MWCNTs in the obtained composite film not only efficiently increase the basal spacing but also bridge the defects for electron transfer between GN sheets, increasing electrolyte/electrode contact area and facilitating transportation of electrolyte ion and electron into the inner region of electrode. Electrochemical data demonstrate that the GN/MWCNT film possesses a specific capacitance of 265 F g⁻¹ at 0.1 A g⁻¹ and a good rate capability (49% capacity retention at 50 A g⁻¹), and displays an excellent specific capacitance retention of 97% after 2000 continuous charge/discharge cycles. The results of electrochemical measurements indicate that the freestanding GN/MWCNT film has a potential application in flexible energy storage devices.     

Hierarchical MnNiCo ternary metal oxide/graphene nanoplatelets composites as high rated electrode material for supercapacitors

For achieving a higher supercapacitor performance, electrode material with high surface area and conductivity such as graphene, graphene nanoplatelets (GNP), and carbon nanotubes along with Transition Metal oxides (TMO) can be used. Herein, the composite of graphene nanoplatelets with ternary metal oxide of manganese, nickel, and cobalt (MNC) is synthesized through a facile cost-effective hydrothermal process and its compositional, morphological, and electrochemical properties are investigated. As-synthesized MNC-GNP composite showed excellent electrochemical properties owing to the high porosity offered by graphene nanoplatelets and synergistic effects produced by individual components of the composite. For comparative studies, ternary oxide MNC was prepared by the same hydrothermal route. The cubic structure of the MNC-GNP composite is confirmed by X-ray diffraction (XRD). Scanning Electron Microscope (SEM) showed distinct hierarchical dendritic structures which showed an increase in density by the addition of graphene nanoplatelets. Electrochemical testing revealed that MNC-GNP exhibited an enhanced specific capacity of 605 mAh g^-1 which is higher compared to MNC which exhibited 243 mAh g^-1 at a current density of 2 mV s^-1. GCD also depicted an increased charge-discharge time in the case of MNC-GNP as compared to its counterpart. MNC-GNP has also shown charge stability up to 99.5 % of capacity retention up to 1000 cycles. Hence, synthesized composite shows to be an effective electrode material for supercapacitors owing to enhanced electrochemical properties.     

Ti3C2Tx-foam as free-standing electrode for supercapacitor with improved electrochemical performance

To prevent restacking of the Ti₃C₂T_x layers, the Ti₃C₂T_x-foam has been successfully synthesized through thermal treatment of Ti₃C₂T_x-film with the hydrazine monohydrate. The interconnected porous structure of Ti₃C₂T_x-foam could effectively reduce the restacking of the Ti₃C₂T_x sheets and shorten the diffusion path of ions and accelerate the intercalation/de-intercalation of ions. The Ti₃C₂T_x-foam-80 used as free-standing electrode achieves a high areal capacitance of 271.2 mF/cm² (122.7?F/g) at a scan rate of 5?mV/s in 1?M KOH electrolyte. It also exhibited a high capability rate of 65.5% from 5?mV/s to 100?mV/s and good cycle life with 88.7% retention of its initial after 10,000 cycles at a scan rate of 50?mV/s.     

A novel high power symmetric ZnO/carbon aerogel composite electrode for electrochemical supercapacitor

We present, for the first time, a new material of symmetric electrochemical supercapacitor in which zinc oxide (ZnO) with carbon aerogel (CA) was used as active material. Physical properties of ZnO/CA composite were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that ZnO has single hexagonal structure and the grain size increases with increase of ZnO compository. The result of cyclic voltammetry indicates that the specific capacitance of ZnO/CA composite in 6 M KOH electrolyte was approximately 25 F/g at 10 mV/s for 2:1 composition. AC impedance analysis reveals that ZnO with carbon aerogel powder enhanced the conductivity by reducing the internal resistance. Galvanostatic charge/discharge measurements were done at various current densities, namely 25, 50, 75, and 100 mA/cm². It was found that the cells have excellent electrochemical reversibility and capacitive characteristics in KOH electrolyte. The maximum capacitance of the ZnO/CA supercapacitor was 500 F/g at 100 mA/cm². It has been observed that the specific capacitance is constant up to 500 cycles at all current densities, which implies that the dendrite formation was controlled.     

Preparation, characterization, and supercritical carbon dioxide foaming of polystyrene/graphene oxide composites

Graphene oxide (GO) was prepared by oxidation of graphite using the Hummers method, and was modified by isocyanate to obtain dispersed GO sheets in dimethylformamide. Polystyrene (PS)/GO composites were prepared by solution blending, and their morphologies and properties were characterized. The addition of GO increased the glass transition temperature of the PS/GO composites. The storage modulus and thermal stability of the composites were also improved compared with PS. Foams of PS and PS/GO composites were prepared by supercritical carbon dioxide foaming. The composite foams exhibited slightly higher cell density and smaller cell size compared with the PS foam, indicating the GO sheets can act as heterogeneous nucleation agents.     

Flexible and freestanding manganese/iron oxide carbon nanofibers for supercapacitor electrodes

In this study, freestanding carbon nanofibers embedded with bimetallic manganese-iron oxide are fabricated for flexible supercapacitor applications via electrospinning. A polyacrylonitrile solution is used as the carbon source, and poly(methyl methacrylate) functions as a sacrificial template to produce microporous carbon nanofibers. The concentrations of manganese acetate and iron acetylacetonate are varied to determine the fabrication conditions for maximal electrochemical performance. The optimized supercapacitor cell exhibits a capacitance of 467 F g^?1 at a current density of 1 A g^?1 and a capacitance retention of 94% at the end of N = 10,000 cycles. The higher mechanical durability of the electrode is confirmed by the absence of electrochemical deterioration at the end of performing 500 bending cycles. Overall, the sample comprising carbon nanofibers, in which the optimal amount of manganese-iron oxide is embedded, has the required pseudocapacitive characteristics and is an interesting choice for high-energy-storage supercapacitor electrodes.     

Fabrication and electrochemical characterization of polyaniline nanorods modified with sulfonated carbon nanotubes for supercapacitor applications

The novel composites of sulfonated multi-walled carbon nanotubes (sMWCNTs) modified polyaniline (PANI) nanorods (PANI/sMWCNTs) were synthesized successfully by in situ oxidative polymerization method in the HClO₄ solution. FTIR and Raman spectra revealed the presence of π–π interaction between the PANI and the sulfonated carbon nanotubes and the formation of charge transfer composites. It was found that the specific capacitance of the PANI/sMWCNT composites was markedly influenced by their morphological structure and the content of PANI which was coated onto the sMWCNT. The specific capacitance of the PANI/sMWCNT composite exhibited a maximum value of 515.2 F g⁻¹ at the 76.4 wt% PANI. The charge–discharge tests showed the PANI/sMWCNT composites possessed a good cycling stability (below 10% capacity loss after 1000 cycles) compared to PANI nanorods.     

Effects of specific surface area of electrode and different electrolyte on capacitance properties in nano porous-structure CrN thin film electrode for supercapacitor

Thickness and specific surface area of the film electrode are critical parameters for supercapacitors. The relationship between the thickness and the specific surface area of the film directly affects the capacitance and electrochemical stability performance of super supercapacitors, which virtually affects the contact chance of ion in the electrolyte on the surface of electrode and the ion transport path of electrode. In this paper, the CrN thin films with a thickness of 200–3500 nm are prepared using direct current magnetron sputtering. Atomic force microscopy (AFM) technique is introduced to investigate the relationship between thickness and the specific surface area of the CrN films. The electrochemical performances of CrN electrode with the nanoporousper structure is analyzed in different electrolytes H₂SO₄, Na₂SO₄ and NaCl aquous solutions. The specific surface area of the film increases linearly with the film thickness increases. The areal capacitance is also linearly related to the specific surface area. The spurtted CrN film with a thickness of 3370 nm has a specific surface of up to 43.59 cm² per cm² footprint area. Its areal and volume capacitances reache to 53.92 mF cm^?2 and 650 F cm^?3 at 5 mV s^?1, respectively. In addition, the areal capacitance of CrN film electrode with 655 nm possesses reaches to 40.53 mF cm^?2 for 0.5 M H₂SO₄ solution, 32.69 mF cm^?2 for 0.5 M Na₂SO₄ solution and 9.17 mF cm^?2 for NaCl solution at a scan rate of 5 mV s^?1. Furthermore, the CrN film electrode exhibits excellent capacitance retention of 95.3%, 93.8% and 89.9% in H₂SO₄, Na₂SO₄ and NaCl electrolytes, respectively, after 2000 cycles. Therefore, the sputtered CrN thin film is an potential electrode material for electrochemical supercapacitors.     

A novel carbon microspheres@SnO2/reduced graphene composite as anode for lithium-ion batteries with superior cycle stability

Many advantages made SnO₂ a potential anode for lithium-ion batteries, but huge volume expansion during cycling seriously impeded its practical application. Here, a novel double-carbon structure with low graphene weight proportion was successfully prepared using a facile hydrothermal method to enhance the long-cycle stability of SnO₂ as anodes for lithium-ion batteries. In this structure, SnO₂ nanoparticles were formed around the surface of the carbon microspheres (CMS), and the reduced graphene (GR) shuttled through the outer layer. As anodes for lithium-ion batteries, the SnO₂ protected by dual carbon (CMS@SnO₂/GR) exhibited outstanding cycle performance with an initial reversible capacity of 789.5 mAh g^-1 and the reversible capacity retention rate of 68.6% after 350 cycles at 200 mA g^-1. The abundance free space among CMS, nano-scale, and the excellent flexibility of graphene were all contributed to alleviating the volume variation of CMS@SnO₂/GR during the lithiation and delithiation.     

High-crystallization polyoxymethylene modification on carbon nanotubes with assistance of supercritical carbon dioxide: Molecular interactions and their thermal stability

As a typical engineering plastic and high-crystallization polymer, polyoxymethylene (POM) has been successfully wrapped on single-walled carbon nanotubes (SWCNTs) using a simple supercritical carbon dioxide (SC CO₂) antisolvent-induced polymer epitaxy method. The characterization results of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) reveal that the SWCNTs are coated by laminar POM with the thicknesses of a few nanometers. The polymer adsorption on CNTs via multiple weak molecular interactions of CH groups with CNTs has been identified with FTIR and Raman spectroscopy. The experimental results indicate that the decorating degree of POM on the surface of CNTs increases significantly with the increase of SC CO₂ pressure, and accordingly the dispersion of SWCNT modified by POM at higher pressure are more excellent than that of obtained at lower pressure. Further the processing stability of POM/CNTs composites are investigated by differential scanning calorimetry and thermogravimetric analysis. The experimental results obtained show that their thermal stability behavior is closely related to surface properties of CNTs. Apparently, the composites with POM-decorating SWCNTs as the filler shows higher melting points compared to the POM composites with pristine SWCNTs as the filler. Therefore, we anticipate this work may lead to a controllable method making use of peculiar properties of SC CO₂ to help to fabricate the functional CNTs-based nanocomposites containing highly crystalline thermoplastic materials such as POM.     

Reduced graphene oxide/hierarchal spinel nickel cobaltite nanoflowers composites for supercapacitor electrodes with ultrahigh capacitive and excellent rate performance

Researchers are extensively investigating transition metal oxides due to their unique porous architectural structure and remarkable electrochemical properties, which are suitable to boost the energy storage capabilities. In present work, facile chemical route was used to synthesize hierarchal spinel nickel cobaltite nanoflowers anchored reduced graphene oxide (NiCo₂O₄-rGO) as high performance electrode material. NiCo₂O₄ anchored rGO demonstrated specific capacitance of 2695 Fg^-1 at 1 Ag^-1, which is greater than pristine NiCo₂O₄ nanoflowers specific capacitance. NiCo₂O₄-rGO showed excellent stability and retention capability of 96% after 2500 cycles at 5 Ag^-1. Furthermore, NiCo₂O₄–rGO exhibited maximum energy density of 93.57 WhKg^?1 at power density of 250 WKg^-1. We have achieved specific capacitance and retention capability which is higher than previously reported results. This enhancement is mainly attributed to the spinel structure of NiCo₂O₄ and its robust structural affinity with rGO. Moreover, rGO possesses extended surface area provided ample of active sites and exceptional synergetic effect which helped to enhance the induction and consequently transportation of e^?/h⁺. More importantly due to its special morphological effects, in future NiCo₂O₄ anchored rGO nanoflowers may open new avenue in research but also used as an efficient electrode material for the construction of high performance supercapacitors.     

Electrochemical reduction of carbon dioxide to ethylene with high Faradaic efficiency at a Cu electrode in CsOH/methanol

The electrochemical reduction of CO₂ with a Cu electrode in CsOH/methanol-based electrolyte was investigated. The main products from CO₂ were methane, ethylene, ethane, carbon monoxide and formic acid. A maximum Faradaic efficiency of ethylene was 32.3% at −3.5 V vs. Ag/AgCl saturated KCl. The best methane formation efficiency was 8.3% at −4.0 V. The ethylene/methane current efficiency ratio was in the range 2.9–7.9. In the CsOH/methanol, the efficiency of hydrogen formation, being a competitive reaction against CO₂ reduction, was depressed to below 23%.     

Disposable screen-printed antimony film electrode modified with carbon nanotubes/ionic liquid for electrochemical stripping measurement

In this paper, two disposable screen-printed antimony film electrodes (SPSbFEs) modified with multi-walled carbon nanotubes (MWCNTs) and ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF₄), respectively for electrochemical stripping measurement are introduced. The modified screen-printed electrode substrate was prepared by homogeneously doping the attractive material into the graphite-based printing ink, and then an antimony film was in situ formed by simultaneously electrodepositing the antimony precursor with interesting analytes on the modified substrate. The electroanalytical performance of the modified SPSbFE for heavy metals based on the anodic stripping protocol was intensively evaluated. It was found that stripping voltammetric measurements of mercury (II) and lead (II) at the modified SPSbFEs resulted in good peaks with very low background contribution. In comparison with the bare SPSbFE, the modifications of both MWCNTs and [Bmim]BF₄ were demonstrated providing more sensitive responses. The results indicated that the SPSbFE modified with 4 wt% [Bmim]BF₄ exhibited well linear behavior in the mercury (II) concentration range from 20 to 140 μg/L (R² = 0.998) with a detection limit of 0.36 μg/L (S/N = 3) under a 120 s accumulation, and good repeatability with a relative standard deviation (RSD) of 4.16% (40 μg/L, n = 12). The proposed electrodes, as new styles of “mercury-free” electrodes, also exhibit encouraging properties for measurements of practical samples.